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December 28, 2010

Microbes Give Ancient Australian Rock Art Their Color

by Sam Savage

A specific kind of ancient rock art found in Western Australia has managed to maintain its vivid colorization and high contrast over more than 45,000 years because it is comprised of living organisms, researchers from the University of Queensland have discovered.

The Bradshaw rock art, which was named after the 19th century naturalist who first discovered them, has been exposed to sunlight and rain for up to 70,000 years and has never been repainted. Now researchers Jack Pettigrew, Chloe Callistemon, Astrid Weiler, Anna Gorbushina, Wolfgang Krumbein, and Reto Weiler have discovered why--the original paint has actually been replaced by what they call a "biofilm" of microbes.

"We surveyed 80 figures in 49 panels of Bradshaw art from 16 different locations in the Kimberley region, on a rough transect from its East to West Coast at around 14-15°S, using portable, digital and measuring microscopes," the researchers reported in the December 2010 edition of the archaeological journal Antiquity.

"We concentrated on Tassel and Sash styles of Bradshaw figures because they are easily recognised, there is little controversy about their classification, and because it is widely accepted that they are derived from the oldest epoch of Bradshaw art," they added.

The University of Queensland team reported that they were able to obtain DNA of 80% of the paintings by using a cotton swab. That DNA, they said, "will be used for later microbial identification and metagenomic sequencing."

"There appeared to be a great variety of micro-organisms when all samples were considered, but a black pigmented fungus, identified as such by yellow fruiting bodies, was prominent," said the researchers.

"Its black pigment made a major contribution to the famous 'mulberry' colored paintings. A reddish organism that could not be resolved into single cells in the field with our portable microscopes, probably a species of Cyanobacteria, was usually found along with the black fungi," they added. "When the black fungi had a minor presence and the red 'cyanobacteria' dominated a particular painting, the overall color was the well-recognized 'cherry' (or 'terracotta') shade rather than the 'mulberry' shade."

The researchers have dubbed the microbes "living pigments," according to BBC News.

"'Living pigments' is a metaphorical device to refer to the fact that the pigments of the original paint have been replaced by pigmented micro-organisms," Pettigrew told the British news agency on Monday. "These organisms are alive and could have replenished themselves over endless millennia to explain the freshness of the paintings' appearance."

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Image Caption: (Scale bar = 1mm. Rectangle left shows approximate location of digital micrograph illustrated right). High magnification view of biofilm in the centre of a Bradshaw painting. Three features are prominent: 1. Cavities, pores and channels etched into the sandstone that would act as microniches for micro-organisms. 2. Black pigmented fungi with yellow fruiting bodies (upper left). The absence of hyphae is consistent with the strangely conservative, rock-adapted Chaetothyriales. 3. Reddish cyanobacteria that may have a mutualistic relationship with the fungi by providing carbohydrate via photosynthesis in return for water. Credit: Antiquity Journal